1. Field of the Invention
The present invention relates to a film formation device for forming a thin film on a substrate such as a glass substrate (for use in a liquid crystal display device, for example) and a semiconductor substrate composed of silicon or the like (for use in a semiconductor device).
2. Description of the Related Art
One type of known film formation device for forming a thin film on a substrate is a plasma device. In a conventional plasma device, a thin film is formed on a substrate as follows: After gas is introduced in a chamber, the gas in the chamber is discharged by means of a vacuum pump until a predetermined pressure (vacuum level) is realized. A plasma is generated while the pressure within the chamber is maintained. From this plasma, a desired thin film is formed on a substrate which is placed in the chamber.
When forming a thin film by a plasma process, the substrate on which the thin film is formed is susceptible to damage due to ion bombardment, electrostatic discharge damage, and the like. Such damage is likely to occur at the beginning or the end of the plasma process, when the plasma is in an unstable state. In order to control such damage, a technique has been proposed which generates a plasma in a high frequency state (e.g., Japanese Laid-Open Patent Publication No. 6-122983).
FIG. 1 shows an example of the above-mentioned conventional plasma device. As shown in FIG. 1, the plasma device 600 includes a chamber 41. A gas mixture is introduced into the chamber 41 through an opening 44 to replace the atmosphere inside the chamber 41. A vacuum pump 46 is used to discharge the gas inside the chamber 41, whereby the pressure therein is maintained at a constant level. A cathode 42 and an anode 43 are disposed so as to oppose each other inside the chamber 41. High-frequency power supplies 49a and 49b having different frequencies are coupled to the cathode 42 via a switch 40, which selects one of the two frequencies in accordance with an instruction from a controller (not shown) provided in the plasma device 600. The chamber 41 is insulated from the cathode 42 with the use of an insulation member 48.
An RF electric field applied between the cathode 42 and the anode 43 generates a plasma 45. The plasma 45 is utilized for forming a film, or performing an etching on a substrate 47 placed on the cathode 42.
The plasma device 600 selectively uses one of the high-frequency power supplies 49a and 49b (by means of the switch 40) at different stages of the plasma film formation or plasma etching process, in an attempt to reduce the damage which likely occurs at the beginning or the end of the plasma process.
However, in accordance with the above plasma device 600, in which the frequency of the supply power is changed during the formation or processing of a thin film, the surface of the substrate 47 (on which the film is to be formed) is still exposed to the plasma. As a result, an interface state is created in the substrate 47 due to irradiation damage. In the case of an MOS type device such as a thin film transistor (TFT), such an interface state present at the interface between a gate electrode insulation film and a channel layer may disadvantageously vary the flat band voltage (V.sub.FB) of the TFT. A variation in the V.sub.FB in turn causes the threshold voltage V.sub.th to vary as shown in the following equation (1): ##EQU1## where .phi.f: a Fermi potential difference K: a relative dielectric constant of the semiconductor
.di-elect cons.: a dielectric constant of a vacuum PA1 q: charge amount of electrons PA1 Na: accepter density PA1 Co: capacitance of the insulation film.
As a result, the device is likely to have an insufficient quality, and hence result in a low production yield.